Supported metal catalysts are typically made by impregnating a suitable support material with a catalytically active metal or with its precursor. For example, catalysts for use in the production of vinyl acetate monomer (VAM) by the reaction of ethylene, acetic acid and oxygen are made by impregnating a support such as silica or alumina with a compound of a Group VIII noble metal such as palladium together with a gold compound and an alkali metal salt, typically in the form of an acetate, the palladium and gold compounds being converted to catalytically active state.
In early examples of fixed-bed catalysts for use in the production of VAM, palladium and gold were distributed more or less uniformly throughout the support, for example, U.S. Pat. No. 3,743,607. Since gaseous reactants do not diffuse significantly into the large fixed-bed catalyst particles, much of the expensive catalytic metal components in the interior of the catalyst were not useful. Subsequently, shell-impregnated, fixed-bed catalysts were developed in which most of the catalytic metals were deposited onto an outer shell of the support particle. For example, Great Britain Patent No. 1,500,167 describes a catalyst in which at least ninety percent of the palladium and gold is distributed in that part of the support particle which is not more than thirty percent of the particle radius from the surface. The palladium and gold being at/near the surface are susceptible to loss through attrition.
In the preparation of shell-impregnated, fixed bed catalysts such as that described in GB 1,500,167 and EP-A-0 569 624, after impregnation of a support with a Group VIII noble metal solution, the noble metal is subsequently precipitated to the support by, for example, treatment with an aqueous solution of an alkali metal salt. Such precipitated noble metal has limited mobility.
U.S. Pat. No. 4,677,084 describes a process for preparing attrition resistant catalyst, catalyst precursor and catalyst support particles and in particular silica-containing vanadium/phosphorus oxide catalysts. The catalyst, catalyst precursor or catalyst support particles are slurried in a solution of an oxide such as silica. The slurry is then spray-dried and calcined to produce microspheres. The process results in the formation of an oxide-rich layer at the periphery of each calcined microsphere.
WO 99/62632 describes preparation of a vinyl acetate catalyst comprising palladium and gold in which it is said that the catalyst contains palladium and gold distributed in a thin shell at or near the surface of the catalyst support. The preparation involves a base fixing step for the palladium but not the gold.
GB 1,521,652 relates to a noble metal-containing catalyst which comprises a mixture of palladium and gold as noble metals, and a support material, and having (a) an outer layer of low or zero noble metal content, (b) an inner shell rich in noble metal and (c) a core having a low or zero noble metal content. According to GB 1,521,652 the geometrical dimensions of the support can, for example, be in the range 1-8 mm. In the preparation of these catalyst materials the carrier material is impregnated with a solution containing palladium salts and gold salts, dried and then an aqueous alkaline solution is added to convert the noble metal salts to water insoluble compounds. After an optional wash and/or drying the material is treated with a reducing agent.
U.S. Pat. No. 4,048,096 relates to catalysts for the preparation of vinyl esters and particularly to palladium-gold catalysts. According to U.S. Pat. No. 4,048,096 reproducing examples of U.S. Pat. No. 3,775,342 produced catalyst with an interior band of palladium-gold alloy deposited on the catalyst support. In the Comparative Example described therein, a catalyst was prepared having palladium and gold deposited in a narrow interior band of approximately 0.1 to 0.2 mm thickness, located approximately 0.5 mm below the surface. The preparation involved treatment with base. U.S. Pat. No. 3,775,342 also describes the use of base to form insoluble noble metal compounds as part of the catalyst preparation process.
There remains a need for an improved metal catalyst composition and in particular, a supported metal catalyst composition.
Thus, according to the present invention there is provided a process for preparing a supported metal catalyst composition which process comprises impregnating microspheroidal support particles with a solution of at least one catalytically active metal, or precursor thereof, such that the metal, or its precursor, is in a mobile state in the support particles, drying the impregnated support particles and then treating the mobile metal, or precursor, in the support particles with a liquid comprising at least one reducing agent to deposit and immobilize the metal, or its precursor, in the support particles such that the metal, or its precursor, is distributed in the support particle in a layer below the surface of said support particle, said layer being between an inner and an outer region, each of said inner and outer regions having a lower concentration of said metal or precursor than said layer.
Also according to the present invention there is provided a catalyst composition comprising microspheroidal support particles having at least one catalytically active metal distributed therein, in which the metal is distributed in the support particle in a layer below the surface of said particle, said layer being between an inner and an outer region of said support particle, and each of said inner and outer regions having a lower concentration of said metal than said layer.
Also, according to the present invention there is provided a composition comprising microspheroidal support particles having at least one precursor of a catalytically active metal distributed therein, in which the precursor is distributed in the support particle in a layer below the surface of said particle, said layer being between an inner and an outer region of said support particle, and each of said inner and outer regions having a lower concentration of said precursor than said layer.
The process of the present invention prepares catalyst compositions which can provide high attrition resistance as well as high activity. The outer region of the catalyst composition may also provide some resistance to poisoning of the catalytically active metal.
An advantage of the process of the present invention is that by treating the dried microspheroidal support particles impregnated with a catalytically active metal, or its precursor, which is in a mobile state in the support particle, with a liquid comprising at least one reducing agent which deposits and immobilizes them, the metal, or its precursor, is distributed predominantly in a layer below the surface of the particle such that the catalyst composition so produced has high attrition resistance as well as high activity.
Preferably, the concentration of catalytically active metal or of its precursor in each of the inner and outer regions is less than half the concentration of the catalytically active metal or of its precursor in the layer.
In a preferred embodiment, the layer containing the catalytically active metal, or its precursor, has an outer edge which is at least 3% and no more than 75% of the particle radius from the surface of the support particle and preferably, at least 5%, and more preferably at least 10% of the particle radius from the surface of the support particle.
Depending upon the size of the support particles, alternatively or additionally, the layer containing the catalytically active metal, or its precursor, preferably has an outer edge which is at least 3 microns and no more than 20 microns below the surface of each support particle, and is more preferably 4 to 20 microns below the surface of each particle, and yet more preferably is 5 to 15 microns below the surface of each particle.
Typically, the layer has an average thickness which is less than half the radius of the particle, for example less than 25 microns. Preferably, the layer has an average thickness of greater than 0.1 microns.
The process for preparing the catalyst composition of the present invention may be used for the preparation of catalysts for use in fluid bed processes, for example, for the production of vinyl acetate monomer.
A suitable support material for use in a fluid bed process is a microspheroidal particulate material. Such particles have a diameter of 1 to 500 microns and are generally spheroidal in shape. When the catalyst composition is to be used in a fluid bed process, as is well known in the fluid bed art, the support particles must be small enough to be maintained in a fluid bed state under reaction conditions while keeping sufficient attrition resistance such that excessive amounts of catalyst composition need not be replenished during the process. Further, although typical particle sizes (as measured by mean particle diameters) should not be so large as to be difficult to keep in a fluid bed state, there should not be an excessive amount of very small particles (fines) which are difficult to remove from the system and may plug gas recycle lines. Thus, typically suitable fluid bed support particles have a distribution of larger to smaller particle sizes.
For example, in the fluid bed manufacture of vinyl acetate from ethylene, acetic acid and oxygen-containing gas, typically, at least 80% and preferably at least 90% of the support particles have mean diameters of less than about 300 microns.
A typical catalyst useful in this invention may have the following particle size distribution:
Persons skilled in the art will recognize that support particles sizes of 44, 88, and 300 microns are arbitrary measures in that they are based on standard sieve sizes. Particle sizes and particle size distributions may be measured by an automated laser device such as a Microtrac X100.
Microspheroidal support particles useful in the present invention are sufficiently porous to permit gaseous reactants to diffuse into the particle and contact catalytic sites incorporated within the particle. Thus, the pore volume should be high enough to permit gaseous diffusion. However, a support particle with an exceedingly high pore volume typically will not have sufficient attrition resistance or will not have sufficient surface area for catalytic activity. A typically suitable microspheroidal support particle has a pore volume (measured by nitrogen sorption) between about 0.2 and 0.7 cc/g. A preferable support particle has a pore volume between about 0.3 and 0.65 cc/g and more preferably between about 0.4 and 0.55 cc/g.
Surface areas (measured by nitrogen BET) for fluid bed support particles with mean diameters and pore volumes useful in the present invention typically are above about 50 m2/g and may range up to about 200 m2/g. A typical measured surface area is about 60 to about 125 m2/g.
Typically useful support particles, especially silica support particles are described in U.S. Pat. No. 5,591,688, incorporated by reference herein. In these supports microspheroidal particles are produced by spray drying a mixture of a silica sol with silica particles followed by drying and calcining. In the preparation, at least 10 wt. %, preferably at least 50 wt. %, of a silica sol is mixed with particulate silica. A useful particulate silica is a fumed silica such as Aerosil(copyright) (Degussa Chemical Company). A typical silica particulate material has a high surface area (about 200 m2/g) with essentially no micropores, and, typically, are aggregates (with mean diameters of several hundred nm) of individual particles with average diameters of about 10 nm (above 7 nm). Preferably, the silica is sodium free. Sufficient particulate silica is added to the mixture to obtain a desired pore volume in the resulting support particle. The amount of particulate silica may range up to 90 wt. % and typically ranges up to 10 to 50 wt. % of the silica in the mixture. Typically, the silica sol/particulate silica mixture is spray dried at an elevated temperature such as between 115xc2x0 to 280xc2x0 C., preferably 130xc2x0 to 240xc2x0 C., followed by calcining at temperature typically ranging from between 550xc2x0 to 700xc2x0 and, preferably 630xc2x0 to 660xc2x0 C.
An advantageous silica sol for preparing a catalyst support useful in the present invention contains silica particles in the sol typically more than 20 nanometers in mean diameter and may be up to about 100 nanometers or more. Preferable sols contain silica particles of about 40 to 80 nanometers. Nalco silica sol 1060 particularly is advantageous because of the relatively large mean silica particle sizes of 60 nm pack less efficiently than smaller sol particles such as Nalco 2327 at about 20 nm. The larger particle size sol yields a final support with higher mesopore volume and less micropore volume.
Although silica-based support particles are the most preferred in this invention, other oxides may be used as long as a particle of appropriate size and with sufficient pore volume is produced in which may be deposited the required catalytic materials. Possible other oxides include alumina, silica-alumina, ceria, magnesia, titania, zirconia and mixed oxides and mixtures thereof. The support may be impregnated with organic or inorganic bases for example Group I or Group II hydroxides and ammonium hydroxide.
Preferably, the catalytically active metal comprises at least one Group VIII noble metal. The noble metals of Group VIII of the Periodic Table of the Elements (IUPAC) are palladium, platinum, rhodium, ruthenium, osmium and iridium. Typically, the noble metal used in a catalyst composition for the manufacture of vinyl acetate comprises palladium. Such a catalyst composition typically contains at least about 0.1%, preferably at least 0.2 wt % palladium to about 5 wt % and preferably up to 4 wt % palladium.
The catalytically active metal(s) may be impregnated in one or more steps onto the support particles in the form of precursor salt solutions. In a preferred aspect of the present invention, microspheroidal support particles are preferably impregnated with a palladium compound in a suitable solvent. Suitable solvents may be water, carboxylic acids such as acetic acid, benzene, toluene, alcohols such as methanol or ethanol, nitriles such as acetonitrile or benzonitrile, tetrahydrofuran or chlorinated solvents such as dichloromethane. Preferably, the solvent is water and/or acetic acid. Suitably, the support particles are impregnated with palladium acetate, sulphate, nitrate, chloride or halogen-containing palladium compounds such as H2PdCl4, which is sometimes also represented as [PdCl2]2HCl, and Group I or Group II salts thereof such as Na2PdCl4 and K2PdCl4. A preferred water soluble compound is Na2PdCl4. A preferred acetic acid-soluble palladium compound is palladium acetate. The palladium compounds may be prepared in situ from suitable reagents.
The catalyst composition suitable for the manufacture of vinyl acetate may also comprise, as promoters, other metals such as gold, copper, cerium and mixtures thereof, preferably gold. These other metals may also be more concentrated in the layer than in the inner and outer regions, that is the inner and outer regions may have a lower concentration of said promoter metal than said layer. Typically, the weight percent of gold is at least about 0.1 wt %, preferably, at least 0.2 wt % gold to about 3 wt % and preferably up to 1 wt % gold. Typically, the weight percent of cerium is at least about 0.1 wt %, preferably at least 0.2 wt % to about 10 wt % or more, preferably up to 5 wt % of cerium. Typically, the weight percent of copper is at least 0.1 to about 10 wt %, preferably up to 5 wt % copper.
Impregnation of the support particles with the gold, copper, cerium or mixtures thereof may be carried out together with or separately from the impregnation of the support particles with the Group VIII noble metal compounds such as palladium compound(s). Suitable gold compounds include gold chloride, dimethyl gold acetate, barium acetoaurate, gold acetate, tetrachloroauric acid (HAuCl4, sometimes represented as AuCl3.HCl) and Group I and Group II salts of tetrachloroauric acid such as NaAuCl4 and KAuCl4. Preferably, the gold compound is HAuCl4. The gold compounds may be prepared in situ from suitable reagents. These promoters may be used in an amount of 0.1 to 10% by weight of each promoter metal present in the finished catalyst composition.
In catalyst compositions suitable for the production of vinyl acetate, in addition to Group VIII noble metals such as palladium and optional promoter selected from gold, copper and cerium, the support particles may also be impregnated at any suitable stage during the preparation process with one or more salts of Group I, Group II, lanthanide and transition metals promoters, preferably of cadmium, barium, potassium, sodium, manganese, antimony, lanthanum or mixtures thereof, which are present in the finished catalyst composition as salts, typically acetates. Generally, potassium will be present. Suitable salts of these compounds are acetates but any soluble salt may be used. These promoters may be used in an amount of 0.1 to 15%, preferably 3 to 9%, by weight of each promoter salt present in the finished catalyst composition.
The impregnation of the support particles may be performed using any suitable technique. A preferable method to impregnate salt solutions is an incipient wetness technique in which there is used a salt solution in an amount up to the volume of the pores of the support particles without excess solution being used. Thus, a desired level of metal compounds such as palladium and other metal species may be incorporated into the support particles by calculating the amount of metals and the volume of solution needed. The impregnation is typically performed at ambient temperature. Elevated temperatures may be used for example, with palladium acetate in acetic acid, greater than 60xc2x0 C. and up to 120xc2x0 C.
The impregnated support particles are dried and optionally, the impregnation step repeated two or more times if there is required higher metal or promoter loadings, than the solubility of the salt in the solvent will allow. The drying step may be performed at up to 140xc2x0 C., preferably up to 120xc2x0 C. The drying step may be performed at ambient temperature and reduced pressure. Air, nitrogen, helium, carbon dioxide or any suitable inert gas may be used in the drying step. The catalyst composition may be tumbled, rotated or agitated by the gas stream or mechanical means to aid drying.
The reducing agent may be a reducing agent such as hydrazine, formaldehyde, sodium formate, sodium borohydride, methanol or alcohols, preferably hydrazine. Hydrazine is preferably used as an aqueous solution.
It has been found that the amount of reducing agent needed to give a layered structure depends on the amount of metal e.g. Group VIII noble metal such as palladium metal which is present in the catalyst composition. Generally, higher concentrations of chemical reagent than have hitherto been used are used, for example  greater than 1% by weight hydrazine. If reducing regents other than hydrazine are used, the molar equivalent to the % by weight hydrazine are used. Thus, for example, if palladium is present in the catalyst composition in amounts of 0.5-2 wt %, a concentration of reducing agent in the liquid in excess of a molar equivalent of 2 wt % hydrazine concentration, for example, at least a molar equivalent of 3 wt % hydrazine, such as a molar equivalent of 4-20 wt % hydrazine and preferably a molar equivalent of 4-8 wt % hydrazine has been found to produce a layered structure of the present invention. It has also been found that the more concentrated the solution of hydrazine or other reducing agent, the greater the distance the layer will be below the surface of the particle. Typically, the reducing agents such as aqueous hydrazine are used at ambient temperatures but temperatures up to 100xc2x0 C. may be used. Typically, an excess of reducing agent is used. The reducing agent will reduce the impregnated metal precursor species to catalytically active zero valence noble metal crystallites.
Preferably hydrazine at a concentration in water of greater than 1% by weight, preferably at least 2% by weight, more preferably in excess of 2% by weight, for example at least 4% by weight is used in the preparation of the catalyst compositions. Thus, according to another embodiment of the present invention there is provided a process for preparing a catalyst composition wherein said process comprises impregnating support particles with a solution of at least one Group VIII noble metal and then contacting the impregnated support with hydrazine at a concentration in water of at least 2 wt %.
Contacting the liquid comprising at least one reducing agent with the mobile metal- or precursor-impregnated support particles deposits and immobilizes the metal or its precursor such that the metal or its precursor is distributed as a layer below the surface of each support particle. Preferably, at least 50% of the metal is distributed as a layer in each support particle. The distribution of the metal may be determined by suitable techniques such as Electron Microscopy.
After the reduction step, the impregnated support particles are preferably washed to remove anion contaminants, for example, nitrates, sulphates and usually halides. For chloride removal, washing with de-ionised water should proceed until a silver nitrate test shows that there is no soluble chloride present. The anion contamination levels should be minimised for the preparation of catalyst compositions suitable for the production of vinyl acetate. Cation contaminants should be minimised for the preparation of catalyst compositions for the production of vinyl acetate; for example to below 0.5 wt %, preferably below 0.2 wt % of sodium in the dried catalyst composition. Low levels of these contaminants are likely to remain; it is not essential that the levels are absolutely zero. On a commercial scale, batch washing may be used. To speed up the process, warm water may be used. Also, ion exchange solutions (such as potassium acetate) can be used to displace chloride and sodium. Also, the reagents used for the preparation can be selected to avoid the use of chloride and sodium, for example, potassium metasilicate instead of other Group I or Group II salts such as a sodium salt.
The support may be impregnated with base. Base may be used to influence the mobility of the metal or its precursor and to affect the size and location of the layer. The base may be added before or during the impregnation of the support with the metal or its precursor. There should not be used so much base as to completely immobilize the metal or its precursor before addition of the reducing agent.
The catalyst compositions of the present invention may be used in fluid bed reactors for the production of vinyl acetate, by the reaction of ethylene and acetic acid with molecular oxygen containing gas in the presence of the catalyst composition. Preferably, a fluid bed reactor is used to produce vinyl acetate under fluidised bed reaction conditions. The reaction temperature suitably is maintained at about 100xc2x0 to 250xc2x0 C., preferably 130xc2x0 C. to 190xc2x0 C. The reaction pressure suitably is about 50 to 200 psig (3 to 14 barg), preferably 75 to 150 psig (5 to 10 barg). In a fluid bed reactor system, the particles of the catalyst composition are maintained in a fluidized state by sufficient gas flow through the system. This gas flow preferably is maintained at a suitable level to maintain the fluidization. Excess flow rate may cause channeling of the gas through the reactor which decreases conversion efficiency. Additional alkali metal salt promoter may be added during the process to maintain activity.